1. Field of the Invention
The present invention relates to a novel process for the production of pulp-like short fibers having a high degree of orientation from an acrylonitrile polymer (hereinafter, referred to as "PAN") without a spinning process.
2. Description of the Prior Art
It is well known that molecular chains of PAN are twisted into an irregular helix due to the strong polarity of nitrile groups in the side chains thereof and have characteristics closely allied to rigid chains. See W. R. Krigbaum et al., Journal of Polymer, Volume XLIII, pp 467-488, 1960. If to such polymers is added a strong polar solvent, such as dimethylformamide, dimethylacetamide, dimethylsulfoxide, or an aqueous NaSCN solution, an aqueous ZnCl.sub.2 solution or an aqueous HNO.sub.3 solution, the nitrile groups attract the molecules of these solvents to combine therewith: thereby, the groups are separated from each other to form a fluid solution, even at room temperature.
A few processes for the production of fibers taking advantages of PAN's properties mentioned above, which comprise spinning a mixed solution of PAN in a solvent through microholes provided in a spinning die, have been known. According to these processes, if the solvent is removed after extruding the fluid solution, PAN is solidified to take into the form of a fiber.
Therefore, the resulting filaments are deemed to be present in the form of a fiber immediately after spinning. However, if the solvent is removed and then the filaments are dried, the PAN molecular chains in the filaments are reconglomerated to form a non-oriented lump after all since the internal molecular chains in the resulting filaments have not been oriented at all. Accordingly, in order to obtain a complete fibrous structure from the viewpoint of the molecular construction, it is necessary to draw the resulting filaments in a high draw ratio of above 5 to 30 so that the molecular chains are arranged in parallel with the axis of the fiber. As the filaments are drawn, the non-oriented, conglomerated PAN molecular chains are become disentangled and extended out while arranging with each other, and thereby form fibers having an extended chain crystal region. Therefore, the drawing process is indispensible in the prior art processes for producing fibers, and the substantial fiber structure, in which most molecular chains are oriented in parallel with the fiber axis, can not be obtained until the resulting filaments are subjected to a drawing process.
However, such prior art processes using a solvent have suffered from the defects that they cost a great deal in extracting, recoverying and purifying solvent and an environmental pollution may be caused due to the use of solvent.
In order to solve such problems encountered in the prior art, various processes which comprise forming a melt by heating a mixture of PAN and water followed by spinning and drawing the resulting melt to produce fibers have been proposed. For example, U.S. Pat. No. 2,585,444 teaches that PAN fibers can be produced by heating PAN hydrate containing 30% to 85% by weight of water to above its melting temperature to give a melted fluid followed by melt spinning the resulting fluid. U.S. Pat. Nos. 3,896,204 and 3,984,601 disclose processes for the production of fibers which comprises heating a mixture of PAN and water of about 20% to 30% by weight to a temperature ranging from 170.degree. C. to 205.degree. C. to give an amorphous melt, and spinning the resulting melt followed by drawing by above 5-folds to form fibers. U.S. Pat. Nos. 3,991,153 and 4,163,770 disclose processes for the production of fibers which comprises spinning PAN hydrate containing 10% to 40% by weight of water at above its melting temperature, that is, a temperature range in which an amorphous, single phase melt of the hydrate is formed, and then drawing the extruded filaments in a 25 to 150-folds draw ratio in a pressure chamber to give fibers. However, according to the prior art processes mentioned above, fibers in which PAN molecular chains are oriented cannot be produced until the resulting filaments are drawn in a high draw ratio, since the spinning step is carried out at a temperature in which the PAN hydrate can exist in the form of a disordered, amorphous melt.
U.S. Pat. Nos. 3,402,231; 3,774,387; and 3,873,508 also disclose processes for the production of fibers for pulp, which comprises heating a mixture of PAN with water of above 100% at about 200.degree. C. to form a melt, and then spinning the resulting melt to produce fibers. However, since in these patents an excess of water is used and the melt of PAN is obtained at a high temperature, the resulting PAN/H.sub.2 O melt takes a random, amorphous form and PAN filaments extruded therefrom also are no more than a non-oriented, continuous foam which practically have neither the orientation of the molecular chains nor the fibrous structure, although they appear to be formed into the form of a fiber externally.
As described above, according to the conventional melt spinning techniques, fibrous structures could not be obtained until the resulting filaments are drawn in a high draw ratio, for example, a 5 to 30-folds draw ratio.